A limit order allows you specify the quantity of the asset to trade at a certain price.
Limit buy orders must have a trigger price lower than the current price, essentially meaning you are waiting for the asset to fall to your preferred entry.
Limit sell orders must have a trigger price greater than the current price, essentially meaning you are waiting for the price to rise up to a level where you would want to sell. This type of order is also commonly known as a take profit.
An example of a limit buy order would be that if you wanted to purchase 1 BTC and expect its price to drop from $6,000 AUD to $5,000 AUD then you would place an order for 1 Bitcoin with a limit price of $5,000 AUD/BTC. This trade will not be executed unless bitcoin drops to or below $5,000 AUD.